1. Field of the Invention
The present invention relates generally to fluid reaction surfaces and more specifically to a turbine blade with blade tip cooling.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Rotor blades used in a gas turbine engine generally include internal cooling air passages to provide required cooling of the blade, especially in the first and second stages. The rotor blades also include seals between the blade tip and an outer shroud of the casing in order to limit the hot gas flow leakage across the resulting gap. A squealer tip is one typical seal in which the blade tip includes a squealer tip rail extending around the blade walls and forming a squealer pocket. Hot gas flow into the pocket and across the gap can also produce damage or reduce the life of a blade. Thus, the blade tip and squealer pockets also require cooling air flow.
One prior art design for cooling the blade tip is shown in FIG. 1. The blade includes a mid-chord serpentine flow cooling circuit which is known as a 1+3 serpentine flow cooling circuit to provide internal cooling for the blade and the leading edge region. The airfoil leading edge is cooled with a backside impingement cooling along with leading edge showerhead film cooling holes and pressure side and suction side gill holes. The cooling air for the leading edge cooling is supplied through a separate radial supply channel. The airfoil main body is cooled with the triple pass forward flowing serpentine cooling circuit that also includes pressure side and suction side film cooling holes and trailing edge discharge cooling holes.
In the cited prior art references, blade tip cooling is accomplished by drilling holes into the upper extremes of the serpentine flow cooling circuit passages from both the pressure and suction surfaces near the blade tip edge and the top surface of the squealer cavity or pocket. Film cooling holes are built-in along the airfoil pressure side and suction side tip sections from leading edge to trailing edge to provide edge cooling for the blade squealer tip. Also, convective cooling holes also built-in along the tip rail at the inner portion of the squealer pocket provide additional cooling for the squealer tip rail. Since the blade tip region is subject to severe secondary flow field, this results in a large quantity of film cooling holes and cooling flow required for cooling the blade tip periphery. FIGS. 2 and 3 show a profile view of the pressure side and the suction side tip peripheral cooling holes for this prior art blade tip cooling design.
For the prior art cooling circuit of FIG. 1, the last leg of the serpentine flow cooling circuit geometry is predetermined by the manufacturing requirement. As a result of the cooling design requirement when the cooling air is bled off from the cavity for the cooling of both pressure and suction side walls as well as the blade tip section, the spanwise internal Mach number for the cooling air flow through the last leg becomes lower. This translates to a lower through-flow velocity and cooling side internal heat transfer coefficient. In other words, the pressurized cooling air delivered into the first leg of the serpentine flow circuit discharges a portion of the cooling through a plurality of trailing edge exit holes, with the remaining cooling air flowing through the blade tip channel where more cooling air is diverted through the blade tip exit holes. Even less cooling air remains to flow down the second leg of the serpentine flow circuit before flowing up the last leg, where more cooling air is diverted out through the film cooling holes on both the pressure side and suction side. After all this cooling air is diverted from the main serpentine circuit, not enough cooling air is left to maintain the high flow rate (Mach number) to provide the necessary cooling to the blade and tip. This lower internal Mach number and low cooling side internal heat transfer coefficient can be eliminated by the use of the serpentine flow and blade tip cooling circuit of the present invention.
The Prior Art reference U.S. Pat. No. 4,753,575 issued to Levengood et al on Jun. 28, 1988 and entitled AIRFOILS WITH NESTED COOLING CHANNELS shows a turbine blade with an internal cooling circuit having a forward flowing serpentine cooling circuit and a leading edge cooling channel that turns at the blade tip and flows into a chordwise extending channel portion (#54 in this patent), the tip channel discharging cooling air through tip holes 59. In the Levengood patent, the serpentine flow circuit does not discharge cooling air to the blade tip as does the circuit of the present invention.
Another Prior Art reference, U.S. Pat. No. 5,403,159 issued to Green et al on Apr. 4, 1995 and entitled COOLABLE AIRFOIL STRUCTURE, shows a turbine blade with an internal cooling circuit having a forward flowing serpentine cooling circuit in which the third and last leg (#92 in this patent) discharges into a blade tip passage (#74 in this patent) with cooling air holes discharging cooling air from the tip passage to the sides and top of the blade tip section. In the Green patent, the cooling air in the serpentine circuit is not discharged onto the tip before flowing through the last leg as in the present invention. However, the first or second legs of the serpentine circuit do not run along the blade tip cap such that the serpentine flow cooling air in the serpentine circuit can be used to cool the blade tip cap as in the present invention.
It is an object of the present invention to provide cooling for a blade tip of a turbine blade without diverting too much cooling air from the internal cooling passages so that proper internal cooling of the blade is still accomplished.
Another object of the present invention is to reduce the cooling air flow requirement while providing adequate blade cooling for a turbine blade.
Still, another object of the present invention is to allow for the first or second legs of the serpentine flow cooling circuit to provide cooling to the blade tip cap without discharging cooling air from the serpentine circuit through the tip cap.
Another object of the present invention is to provide for individual blade tip cooling flow circuit on the pressure side and on the suction side that can be selectively sized for cooling flow.